The developing trend of the AC/DC, DC/DC converters is high efficiency and high power density. Through the Resonant AC/DC, DC/DC Converters it is easier to achieve high efficiency due to its feature of soft-switching.
However, there are still certain existing drawbacks regarding the Resonant Converters, e.g., the high AC-current of the output filter resulting in the high power losses and the large volume of the output filter. Furthermore, the center tapped transformer is less efficient and more difficult to optimize due to the passive winding.
The most widely used Series Resonant Converter found in the industry is the LLC-Resonant Converter, due to its availability to achieve high efficiency. This topology has been described in different publications, an example of this is the publication “Steady-state analysis of the LLC Converter”, IEEE 16th Annual Applied Power Electronics Conference, Volume: 2, pp: 728-725, March 2001 by James F Lazar, Robert Martinelli.
The current protection of the circuit disclosed in this publication has been analyzed and the solution using splitting resonant capacitors and clamping diodes has been presented in the publication “Over Current Protection Methods of the LLC Resonant Converter”, IEEE 18th Annual Applied Power Electronics Conference, Volume: 2, pp: 605-609, February 2003, by Bo Yang, Fred Lee, Matthew Concannon.
For low voltage/high current applications and for high power applications in particular, it has been known to place two or more Resonant Converters in series and/or parallel, as described in the publication “Phase-Staggering Control of a Series-Resonant DC-DC Converter with Paralleled Power Modules”, IEEE Transactions on Power Electronics, Volume: 3, pp: 164-173, April 1988, by Ben Klaassens, W. L. F. H. A. Moize de Chateleux, M. P. N. Van Wesenbeeck.
The present invention uses the concept of the so called Three-State Switching Cell (TSSC) in a novel manner, however, the Three-State Switching Cell is part of the prior art of the present invention and it is described in publication “Generation of a family of non-isolated DC-DC PWM converters using a three-state switching cell”, IEEE 31th Annual Power Electronics Specialists Conference, Volume: 2, pp: 858-863, 18-23 Jun. 2000, by G. V. T. Bascopé and Ivo Barbi.